It's a brush made to look like LED lights similar to what could be seen on gaming accessories, on amusement park rides at night, etc.
For best results, have a black background below the layer you're drawing on. Drawing directly on a black background will not result in the same colors appearing.
Tip: It's sorta optical illusion based. From a distance, you should get the impression there's a full spectrum of color appearing, including purple, orange, teal, etc. So, if your stroke looks like red, blue, green with white lines, your brush size is either too big, or you're too zoomed in. Zoom out/size down to desired results.